1. Field of the Invention
The present invention relates to a polyurethane foam and a pneumatic tire including the same, and more particularly, to a polyurethane foam which can reduce resonance noise generated in the cavity of a pneumatic tire, and a tire produced by applying the polyurethane foam.
2. Description of Related Art
The resonance noise generated in the cavity or the internal space of a tire is an air-borne noise generated in the hollow cavity present inside a tire, and recently, more interest is being paid to an improvement of such noise in the fields of automobile industry and tire industry.
Along with the emergence of electric vehicles, more attempts have been made to improve the driving quality of automobiles. Under such circumstances, there is an increasing demand for reductions in the interior noise as well as the exterior noise attributable to the tire treads, in order to improve the riding comfort and to attain pleasant driving environment.
Among these noises, the noise attributable to the internal space or cavity of a pneumatic tire causes an unpleasant feeling to the driver and deteriorates the driving comfort, because this noise is generated as a sound having sharp peaks in the interior of vehicles.
As electric vehicles that produce very little or no engine room noises have emerged, and the configuration standards for the tires mounted in vehicles have been directed toward less flattening and inch-up, such tire cavity resonance noise is increasingly considered as an important issue.
According to the related art technologies for reducing such tire cavity resonance noise, a special sponge having a sound absorption performance is provided on the inner surface of tires (Korean Patent Application No. 2006-0125502), or the interior space is modified by disposing an annular body that can be mounted over the rims inside a tire and a wheel (Korean Patent Application No. 2005-0102110). In another case, the noise due to the resonance of the tire cavity is reduced by attaching a material such as fur on the surface of the wheel.
However, when a special sponge having a sound absorption performance is provided inside the tire interior, or a foreign material is attached on the wheel surface, additional processes should be added after the production of the tire and the wheel, so that the production cost increases, and the storage of produced tires and wheels may not be as easy as expected. Also, when such a sponge or foreign material is attached over the entire perimeter, the rolling resistance performance (RR performance) may be deteriorated as a result of an increase in weight.
There has been another suggestion that spherical sound absorbing bodies having a diameter of 40 to 100 mm are inserted inside a tire without fixing to the tire wall (Japanese Patent Application No. 2002-240507), so that the sound absorbing bodies can reduce the tire cavity resonance noise. It is considered that such spherical sound absorbing bodies are usefully compliant to the general PCR standards, and adhesion for preventing heat generation is not used. However, since the recent configuration standards for tires are directed toward less flattening, the internal space is narrowed, and accordingly, it may be difficult to apply such spherical sound absorbing bodies. Also, there is still room for improvement in terms of the durability of the sound absorbing bodies.
In view of the material of the sound absorbing bodies, conventional soft foams which are applied to the dashboard unit that separates the engine room and the interior of the vehicle, do not have sufficient sound absorption performance in the low frequency region when used alone. Therefore, there has been an attempt to improve the car sound performance by attaching a thermoplastic material such as a rubber containing a polyolefin as a main ingredient, to a soft foam or the like (Japanese Patent Application Laid-Open No. 5-209036). However, this problem involves another problem of weight increase and the like because it is necessary to consider the adhesiveness of the soft foam and the thermoplastic material. Thus, it has been suggested to improve the sound absorption characteristics in the low frequency region by employing a soft polyurethane foam as the material for the sound absorbing bodies (Korean Patent Application No. 2006-0059957). In this case, a raw material composition containing a high molecular weight polyoxyalkylene polyol, an organic polyisocyanate compound, an expanding agent and a catalyst is used, and this raw material composition is expanded within a closed mold so as to control the air permeability to 0.085 m3/min or less.
Furthermore, a technology for providing a soft polyurethane foam for tires which is mounted on the inner surfaces of a tire by means of a band member, and functions effectively to suppress damage such as cracks, or deformation such as compression (collapse), has also been introduced (Korean Patent Application No. 2008-0070862). However, the performance of this soft polyurethane foam in connection with the reduction of resonance noise in the tires provided with the foam is not considered effective.
Furthermore, according to Korean Patent Publication No. 2010-0047024, in order to attain characteristics such as a soft feeling to touch, a high density, high air permeability, and a high resilience, there is provided a soft polyurethane foam which is obtained by a reaction between a polyol component and a polyisocyanate compound. In this soft polyurethane foam, the polyol component contains a polyol compound, water as an expanding agent, a catalyst, and surfactants, and the polyol compound is a copolymer of propylene oxide and ethylene oxide having an average number of functional groups of 2 to 4. The polyurethane foam is composed of a polyether polyol compound having an ethylene oxide content of 50% to 90% by weight and a hydroxyl group value of 30 to 70 mg KOH/g, and a polymer of propylene oxide and ethylene oxide, and the copolymer occupies 50 to 90 parts by weight relative to 100 parts by weight of the polyol compound.
However, when these conventional soft polyurethane foams are used alone, they do not sufficiently reduce the resonance noise in the low frequency region, and polyurethane foams themselves have low rigidity, so that it has been difficult to overcome problems such as deformation or destruction due to the low rigidity.